ABSTRACT: Many common diseases, such as cancer, diabetes, and cognitive decline have all been linked to advanced age, and a large portion of biomedical research is dedicated to unraveling the complex biological changes that link aging to disease. Recent studies have shown that age can be estimated?independent of sex or tissue type?based on the methylation status of 353 CpG dinucleotides in the nuclear genome. The weighted average of these sites forms an age predictor (or ?DNA clock?) called the Horvath Clock, after its discoverer, Dr. Steve Horvath. Additional work with the Horvath Clock has indicated that there is an ?age acceleration? in the clock for a variety of age-related conditions such as Alzheimer's disease and premature aging syndrome. The mechanisms by which these methylation changes occur during aging remain unclear, but several studies have shown an interaction between the nuclear and mitochondrial genomes relative to the epigenetic landscape. Our recent studies have demonstrated that mutations in the mitochondrial genome (mtDNA) are acquired rapidly, leading to defects in mitochondrial metabolism and an increased production of reactive oxygen species (ROS). Other research has shown that increased ROS due to mitochondrial dysfunction can affect methylation in the nuclear genome. Based on this and similar results linking ROS levels with DNA methylation changes, we hypothesize that the mitochondrial dysfunction caused by age-related mutations in mtDNA results in increased ROS production and is linked to epigenetic markers of aging. We propose the following approaches to explore the role of mitochondrial dysfunction in modulating the DNA clock: 1) To examine the effects of known pathogenic mtDNA mutations in the DNA clock. One popular model for aging in mammals is that the aging process is fundamentally caused by a vicious cycle of mitochondrial mutation, wherein mtDNA mutations trigger respiratory chain dysfunction, resulting in increased ROS that causes additional mtDNA mutations and a further loss of respiratory chain function. We propose using a commercial microarray platform which includes all 353 age-related CpG sites to investigate whether there is accelerated DNA methylation age in patients harboring pathogenic mutations in mtDNA compared to sex- and age-matched healthy individuals. 2.) To examine the effect of ROS on accelerated epigenetic age in a mouse model. Age-related loss of physiological function is, to a large extent, due to the progressive accumulation of oxidative damage caused by increased mitochondrial ROS and progressive decline in antioxidant function. ROS-induced DNA methylation could be an important mechanism for the epigenetic regulation of gene expression, making cellular mechanisms for neutralizing ROS essential. We intend to investigate whether disruptions in the normal production and degradation of ROS in the cell can lead to a quantifiable acceleration in the DNA clock. We propose to test this hypothesis using a mouse model of ROS- induction via injections of D-galactose, and to measure the resulting shift in epigenetic aging.